STS-133: The battle continues for ET-137 flight rationale

by Chris Bergin

Despite the large amount of progress made towards flight rationale – called for in relation to Discovery’s External Tank (ET-137) stringer cracks – teams are heading into another week of deliberations, with a large amount of work ahead of them. Although STS-133 is threatening to move into next year, NASA managers will continue to evaluate the opportunity to make a late December window.

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Last week’s Program Requirements Control Board (PRCB) meeting was – at least in the eyes of some sources close to the situation – set to show “a path through the trees” on enabling NASA managers to be satisfied the repaired ET-137 would be safe to fly.

However, in a sign of just how seriously the Agency and its contractors take safety, the cautious approach of reaching deeper into the root cause findings, along with further evaluations into the potential ascent hazards, was opted for early into Thursday’s meeting.

With ET-137 now in a repaired state at the pad, managers were hoping to take a launch window opportunity which ranged from December 3 to December 7. With the decision to delay, the realigned target was pushed to the next window, ranging from December 17 through to December 20 – although this window is simply for planning purposes at this stage.

“There is still significant open work to understand root cause and risk associated with stringer cracks. The schedule of open work reflected an ECD (Estimated Completion Date) of 12/15. Therefore, the Dec 3rd-7th launch window is not feasible. For planning purposes, launch is assumed to be NET (No Earlier Than) 12/17,” noted the PRCB summary (via L2).

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As noted by Space Shuttle Program (SSP) manager John Shannon, an internal status update will be conducted at the next scheduled PRCB meeting this coming Thursday, prior to updating NASA management on the Friday. At present, the procedure towards the Launch Status meeting – a mini Agency Flight Readiness Review (FRR) to confirm a new launch date – will remain on hold for at least a week.

As noted in presentations to the November 18 PRCB, progress was being made on flight rationale, although a lot was riding on finding issues specific to the two cracked stringers on ET-137, namely S6 and S7. This proved to be unsuccessful by the time managers met prior to Thanksgiving.
“Proximate root cause for the stringer cracks has not been identified. Initial evaluation of the crack locations and surfaces indicate that there was not a pre-existing flaw. However, additional work is planned to determine whether the crack could have initiated during installation and then propagated under cryo loading,” added the November 25 PRCB notes.

“No issues were identified during disassembly/re-assembly of S6 and S7 (e.g. shims not installed originally and were not required, smooth flat skin with no defects noted). There are 91 items identified in the fishbone analysis, 21 of which are considered high interest items. 

“Since the stress models show that a “good” stringer would not fail in the manner observed, manufacturing/installation induced defects/stresses are considered primary contributors. However, additional work is planned to ensure the models are sufficiently accurate to conclude a “good” stringer would not crack if properly installed.”

The cracked stringers were first noticed via a crack in the tank’s Thermal Protection System (TPS) foam, when cameras spotted the foam defect localized on an area of the LOX/Intertank flange.

While the foam crack was noticed during the draining of ET-137 – following the scrub relating to the leak on the Ground Umbilical Carrier Plate (GUCP) – it was only after the foam was removed that the four cracks were found in the two stringers.

A question being worked relates to if the cryo process of tanking provides the highest amount of stress on the stringers – thus allowing engineers to be confident with status of the stringers prior to launch if no defects are detected, or if higher stresses during ascent (such as MaxQ) could result in stringer defects propagating and ultimately leading to foam liberations – that in turn may impact on to the orbiter.

“To determine whether tanking is a good screen for this failure mode, thermally induced stresses during cryo loading were compared to in-flight loads: Initial analysis results show the stress during cryo loading is higher than in-flight for the region of about the first 4 fasteners but flight loads are predicted to be higher than cryo loads for the region of the 5th through 8th fasteners,” continued the PRCB notes.

“Since certain simplifying assumptions were made, a higher fidelity assessment will be performed.

“Analysis to demonstrate structural fault tolerance is in work. Preliminary debris analysis indicates that stringer debris is not expected but TPS liberation is possible. However, this analysis assumed the TPS was not bonded. A higher fidelity assessment will be performed.

Large sections of the flange also underwent Non Destructive Evaluations (NDE) at the pad, via in-situ scanning techniques – with results showing no other defects have been noted in other stringers on ET-137, although the PRCB did note that the critical flaw size and probability of detection have not been determined.

Some areas of the flange also remain outstanding due to the limitations of pad access, while engineers had already begun scans on the lower flange areas, located between the intertank and the LH2 tank.

With the teams now back after the Thanksgiving break, a list of “significant open work” has been created, ranging from “mining” for historical crack data on the root cause drive, to rechecking ascent imagery in an attempt to catch for any cracks that may have formed in the flange area during the ride uphill – although it has been previously noted that no notable foam liberations have originated from the LOX/Intertank flange region.

“Previous crack data mining and how assembly/installation may contribute. Higher fidelity test fixture (utilizing complete stringer sections instead of just coupon testing), testing with flaws. The objective is to induce cracks to better understand the failure mechanism,” listed the PRCB notes.

“Higher fidelity stress analysis for prelaunch cryo loading and in-flight stresses to understand ability to screen with tanking. Fishbone work to drive out areas that require additional investigation. Structural fail safe analysis (e.g. how many stringers can be lost).Higher fidelity debris analysis for structure and for TPS (e.g. with foam adhered).

“KSC (Kennedy Space Center) evaluation of access for making modifications if deemed necessary. SE&I (Systems Engineering and Integration) relook at imagery to look for U shaped type cracks.”

(Further updates and articles will follow. Refer to live coverage threads linked above. L2 members refer to STS-133 live coverage sections for internal coverage, presentations, images and and updates from engineers and managers. Images used, Lead: Within the article: via L2 acquired PRCB presentations).

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